Such angle-measuring devices are often used for determining the angular position of two machine parts rotatable relative to each other and, for example, operate according to an optical measuring principle. For instance, a rotor of a wafer-inspection system is considered as a rotatable machine part. Such systems are usually operated in clean rooms.
PCT International Published Patent Application No. WO 2005/010940 describes such a wafer-inspection system, which has vacuum connections for removing contaminants by suction. The system also includes an angle-measuring device.